This invention relates generally to rinsing or sealing metal surfaces subsequent to conversion coating, and more particularly concerns improving the corrosion resistance of conversion coated surfaces with a solution that minimizes disposal problems.
In the metal finishing industry it is well known to use a zinc phosphate or iron phosphate conversion coating on a metal surface to promote adhesion of paint or other finishes. The corrosion resistance of conversion coatings may be improved by the use of a rinse subsequent to conversion coating with a solution for sealing the conversion coated surface. Rinses including chromic acid or a chromate compound are known to be effective when used as rinse additives to seal the metal and improve corrosion resistance of the conversion coating. The primary problem with the use of chromate final rinses is their classification as toxic chemicals. Chromium (III) final rinses have been developed and are preferred due to their lower toxicity. The primary inpetus for developing non-chrome rinses is the problem of disposing of waste residue in a safe manner.
Considerable work has been done to develop a non-chromium sealer for conversion coated surfaces. Polymer-based sealers have been proposed but have generally not been as effective in terms of corrosion resistance and cost effectiveness as chromate sealers. Another approach has been to apply a solution of non-metallic ions to a conversion coated surface. Non-metallic ion solutions proposed include solutions of phosphates, phosphonates, flourides and silica with certain quaternary amines. Another alternative to the chromium final rinses that has been proposed is the use of a solution of metal ions such as zirconimum, titanium peroxide, molybdenum, aluminum, permangenate and the rare earth elements. The above proposed replacements for chromium final rinses have been less effective than chromium final rinses when tested in cyclical tests and other accelerated corrosion tests that are generally considered more realistic than salt spray testing.
In addition to corrosion resistance, with modern automobile assembly line paint systems it is important that a sealer following phosphate be effective in promoting adhesion when used with cathodically electro-deposited paints. Some of the above replacements for chromium final rinses have not provided equivalent paint adhesion improvement when used with a cathodically electro-deposited paint system.
The present invention is directed to overcoming the problems set forth above.